This application claims the priority of Korean Patent Application No. 2003-91332, filed on Dec. 15, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a passive optical network (PON), and more particularly, to the PON and a media access control method for the same.
2. Description of the Related Art
In order to provide data service to subscribers using typical telephone lines, x-digital subscriber line (XDSL) technology has been developed and used. Besides, a cable network using coaxial cable has been suggested as a plan for another subscriber data service. When taking current Internet traffic of subscribers into consideration, there seems to be no problem till now. However, when intending to popularize future very high speed services such as small office home office (SOHO), a teleconference service, a high definition television (HDTV) class video service, tele-education, and telemedicine, it is predicted that it is difficult to fully provide wideband and high quality services using current networks with band and distance limitation.
To fulfill the future very high speed services, optical communication technology, such as fiber-to-the-curb (FTTC) and fiber-to-the-home (FTTH), is suggested, and PON technology is coming to the front as a subscriber access method for providing wideband services required by users. The PON forms a tree-shaped distributed topology sharing a link between an optical line termination (OLT) and a passive optical splitter and diverging from the passive optical splitter to subscribers using a plurality of optical network units (ONUs) and is suitable for broadcasting services. That is, in a downstream service, a data broadcasting service is smoothly achieved according to characteristics of the passive optical splitter, however, in an upstream service, contention occurs in the shared link between the OLT and the passive optical splitter. To prevent the contention, a time division multiplexing (TDM) method, in which mutual communication is performed not so as to be overlapped by temporally dividing a same frequency band, is used for an asynchronous transfer mode (ATM)-PON and an Ethernet-PON (EPON).
FIG. 1 illustrates data transmission in a PON based on TDM. FIG. 1 shows a general solution for providing an FTTH service based on one carrier.
Referring to FIG. 1, A passive optical splitter is placed between service providers (SPs) connected to an OLT via an external network and subscribers connected to optical network terminals (ONTs), and packets provided from the SPs are multidropped and transmitted to a plurality of ONTs. Here, the downstream packets are smoothly diverged in an optical domain, optical-electronic converted in the plurality of ONTs, and filtered using addresses of the packets or unique IDs (for example, LLID of EPON). Upstream packets are separately transmitted during timeslots with a predetermined time period at time allocated for each ONT. At this time, for downstream packets from the OLT to the ONTs, no sharing method is necessary, however, for upstream packets from the ONTs to the OLT, a TDM method is necessary.
That is, a TDM-PON has strong points in optical cable installation, expandibility, and economical efficiency, which are advantages of a PON service, does not require special control for a downstream service since the TDM method is used, and makes a high speed downstream service possible. However, the TDM-PON can make problems due to complexity of the TDM method or a TDM delay of packet transmission. Also, capacity of packets in a second layer of Open Systems Interconnection (OSI), which should be processed in the ONT for each subscriber, may be so large that cannot be processed with a link speed between the OLT and the ONT.
FIG. 2 illustrates data transmission in an active optical network (AON) connected to subscribers via an active node. A switching method shown in FIG. 2 is an active switching method called a home-run method.
Referring to FIG. 2, the active switching method divides and uses a physical link between an OLT and ONTs via the active node using a point-to-point method. The active node should find out a target port through optical-to-electrical conversion and perform optical transmission to the ONTs or the OLT through electrical-to-optical conversion.
In the AON, each user's ONT is independent, and downstream and upstream services can be provided using a same method. However, the AON is worse than the PON in optical cable installation, expandibility, and economical efficiency, and in the AON, there is overhead of the optical-to-electrical conversion and the electrical-to-optical conversion in subscriber distribution and connection.